The Experts: The notch effect

Q: What is the notch effect and how does it affect bolts?

A: A notch is basically each change of the cross-section of a component. It may come from drill holes, grooves or cross-sectional variations. Notches result in uneven stress distribution, leading to stress peaks – known as the notch effect. The notch effect reduces the loading capacity, which makes it a crucial parameter when calculating the mechanical strength of components or constructions. It especially appears when the component is under tensile, compressive, bending, shearing or torsional stress. The notch effect – or the stress peaks – can lead to early failure, so it is normally considered a negative impact. A solution is to design the component sufficiently large.

But notches are not all bad. They can be designed to strengthen a component, for example strain hardening at the bottom of a thread, or to create a predetermined breaking point.

Bolts under tensile stress have local stress peaks due to their notch effects. The tensile breaking stress, especially of ductile material, can be up to 20 percent higher than that of un-notched round bars, due to the prevention of lateral contraction.

The notch effect in the free loaded thread of a bolt is not as big as in the thread run-out, or in the first loaded thread, due to the relieving notch effect of strung-together threads. Under tensile stress, a bolt will break in the free loaded thread, fulfilling the design principle that a bolt is supposed to break there. There is normally a ductile deforming of the threads quite early, so you can recognize the failure in time.

Besides tensile stress, bolts are normally also under repeated loads, which is important for their loading capacity. The notches generate high notch stress, which are again the reason why bolts can’t transfer vibrational loading. It could even lead to fatigue fracture of the bolt.

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